The IOp keyword allows the user to set internal options (variables in system common /IOp/) to specific values. The syntax is:
IOp(overlaya/optiona=valuea [,overlayb/optionb=valueb, …])
which sets option number optiona to the value valuea for every occurrence of overlay numberoverlaya (and so on for other entries in the list). For example, the following sets the value of option 12 in overlay 1 to 5 and that of option 44 in overlay 3 to 0: IOp(1/12=5,3/44=0)
IOp values explicitly set in the route section are not passed on to the second and subsequent automatically-generated job steps; this applies to keyword combinations like Opt Freq and to inherently multi-step methods such as G2 and the CBS methods. For example, if you want to specify an alternate grid for a DFT optimization+frequency job, you must use an option to the Int=Grid keyword rather than an explicit IOp value.
The execution of each overlay of Gaussian 16 is controlled by numbered options. Each option may be assigned an integer value, with 0 being the default. The value of an option is held unchanged throughout execution of all of the links in one overlay. Thus the significance of a particular option applies to all the component links in one pass through the overlay. Since setting internal options can have arbitrary effects on the calculation, archiving is disabled by use of this keyword.